The following paragraphs are intended to introduce the reader to the more detailed description that follows and not to define or limit the claimed subject matter of the present disclosure.
Vaccines capable of mediating an effective immune response are important in health strategies aimed at combating diseases caused by microbial pathogens. The two basic strategies for inducing an effect immune response in the host involve either the administration to a subject (host) of a ‘live’ agent capable of replicating within the host, or the administration of materials or substances that are not capable of replicating in the host. Administration of a live vaccine may represent a safety risk for immune-compromised individuals if the agent or a contaminating organism replicate and adversely affect the immunized subject. These risks are not associated with vaccines based on killed whole pathogens, based on extracts from pathogens or based on purified components, commonly referred to as subunit vaccines. Subunit vaccines avoid the safety problems associated with live vaccines but the purified components may not by themselves deliver the desired protective effect in the subject against the infective microbial organism and require appropriate components, termed adjuvants, to enhance the immune response.
Approaches to the design of vaccines against Gram-negative bacterial organisms have commonly focused on the use of proteins that are naturally associated with the outer membrane of the bacteria and exposed on the surface of the bacterial cells. Particularly attractive targets for vaccination are proteins presumed to be critical for survival in the host as they cannot be lost or dramatically altered in order to avoid the host immune response. In this respect the bacterial surface receptor proteins capable of interacting with and binding to the host iron binding proteins, transferrin and lactoferrin, have for some time been considered suitable components for use in the preparation of vaccines (1-3). This group of surface receptor proteins, hereinafter referred to as “HIBP” (host iron binding protein) surface receptor proteins, is present in pathogens of humans and animals belonging to the bacterial families Pasteurellaceae, Moraxellaceae and Neisseriaceae (4). Thus these proteins have been recognized as potential targets for development of vaccines against a variety of different pathogens of humans and food production animals (5) (6-10).
The HIBP surface receptors normally are comprised of two proteins, a surface lipoprotein, transferrin binding B (TbpB) or lactoferrin binding protein B (LbpB), and a TonB-dependent, integral membrane protein, transferrin binding protein A (TbpA) or lactoferrin binding protein A (LbpA) (11). Recently the detailed three-dimensional structures of TbpBs from Actinobacillus pleuropneumoniae, Actinobacillus suis, and Neisseria meningitidis were determined at high resolution (12-14). The intrinsic properties of the TbpB or LbpB proteins are quite different from the integral outer membrane proteins, TbpA and LbpA, and substantially impact on the strategies used for vaccine development. For instance, it is possible to produce and purify TbpB or LbpB at relatively high yields from the E. coli cytoplasm for the generation of subunit vaccines. However, these proteins are notably absent or deficient in outer membrane vesicle (OMV) vaccines prepared by selective detergent extraction due to their removal during the extraction process. In contrast, functional TbpA or LbpA can only be produced in the outer membrane, providing limitations for producing high yields of purified proteins to be used in subunit vaccines. The alternate approach of producing misfolded proteins that aggregate into large inclusion bodies and subsequently attempt to refold the protein from the enriched inclusion body preparations, are also problematic for commercial production. Thus most strategies for TbpA or LbpA based vaccines would normally involve production of OMVs or development of attenuated strains.
An alternate approach that has been used successfully for invasive bacterial pathogens is to use the extracellular capsular polysaccharide as the primary antigen, and couple it to a protein carrier to induce T-cell help. These conjugate capsular vaccines have proven to be very effective at providing protection from infection by strains expressing the specific capsular polysaccharide but provide no cross-protection to other capsular types. Although conjugate capsular vaccines against the human pathogens H. influenzae, N. meningitidis and Streptococcus pneumoniae were originally developed to prevent invasive infection, post licensure carriage studies have demonstrated that the systemically administered vaccines eliminated colonization by the pathogens expressing the specific targeted polysaccharides (15-17). This had the added benefit of providing herd immunity, providing protection to non-immunized individuals due to reduced carriage frequency within the population. While not the initial intent of these early vaccines, the potential to confer herd immunity has become an important criterium for evaluating new and upcoming bacterial vaccines. However, determining or predicting whether new vaccines will be capable of impacting or preventing colonization (carriage) is a major challenge (18).
The ability to evaluate whether protein antigens will ultimately be capable of providing broad protection against a diverse set of disease isolates is also a considerable challenge (19). Initial efforts at testing this ability commonly involve immunizing other animal species (mice, rabbits) and then analyzing the cross-reactive and cross-protective properties of the resulting sera. For those surface antigens that can readily be produced in a soluble form, such as surface lipoproteins, a first step is often to produce and purify a set of variant proteins and use them in a standard ELISA (enzyme linked immunosorbent assay) to evaluate the ability of the antisera to recognize the variant proteins. This is fairly labor intensive making analysis of an extensive set of variants an expensive enterprise and relies on the assumption that the binding of the antigen to the ELISA plate is random so that all protein surfaces can be probed.
Considering the limitations in the various assays used to evaluate and predict the cross-protective and cross-reactive properties of antisera raised against antigens, the selection and design of new and improved protein-based vaccines should be pursued in conjunction with development of improved assays so that optimizing and improving vaccines in development can be approached on a rational basis.
Despite considerable efforts over the years since their initial discovery (20, 21), it remains unclear whether and how HIBP surface receptor proteins can be used to prepare efficacious vaccines against Gram-negative bacterial pathogens, and in particular whether and how a broadly protective vaccine can be developed. Thus there is a need in the art to improve immunogenic compositions and vaccines based on HIBP surface receptor proteins against Gram-negative bacterial organisms.